The FL Series of Ferranti High Speed on-Load Tapchangers (tap changers) are resistor transition equipments which are intended for use at the line end of 132kV transformer windings. The tap changers are designed to give reliable service under the most arduous conditions which occur on electrical power systems. No special lubrication is required.
The equipment is designed for a maximum number of 19 tapping steps, i.e. 20 positions.
The type FL Ferranti Tapchangers (tap changers) are virtually single-phase units and three single phase units are required for three-phase operation. read more.... They have electrical characteristics which ensure that the transformer gives satisfactory performance both on test and in service. Apart from exceptional cases, the insulation strength is sufficiently high to avoid any necessity for non-linear resistors to be connected across the tapping winding of the transformer. The selector switch is of the linear type, and high pulse input voltages cannot be developed across adjacent contacts. A complete tapchanger (tap changer) arranged for three phase working is shown above. Each single phase switch assembly is arranged for bolting direct onto the transformer tank and comprises the diverter switch compartment, bushing assembly and selector switch unit. The divertor switch compartment is mounted on the top of the 132kV outdoor type bushing and the moving arm of each diverter switch forms the 132kV line terminal. The oil in the diverter switch is separate from that in the selector switch compartment and bushing assembly, and also from the oil in the main transformer tank. The selector switch compartment is mounted underneath the 132kV bushing and the method of connection between the transformer windings is via the rear of the selecter switch. The selecter switch mechanism for each phase is supported on a large barrier panel insulator which is made from epoxy resin and provided the basic 132kV insulation. This panel, together with the 'bath tub' style barrier board switch base , forms an oil tight barrier between the transformer tank and the selector switch compartment., and this barrier is capable of withstanding a vacuum. Busbars carry the current through an aluminium stress tube which forms the central electrode of the 132kV bushing, and which provides the connection to the divertor switch mounted on top of the bushing porcelain. The Ferranti Tapchangers Limited type FL Tap Changer can be supplied with an internal condenser bushing which gives provision for the inclusion of current transformers. High speed stored energy diverter switches which are fitted in each housing provide long contact life in service. Separate wire wound resistors for transition for each half of the divertor switch give effective current division on the transitional contacts and so increase the reliability on the switch. All three switch assemblies are driven from a common gear box and timing gear assembly, and the method of coupling between the single phase units is via drive shafting. Voltage variation is achieved by providing the full number of tappings on the transformer windings. Three different arrangements of tappings are possible.
The Driving Motor and Timing Gear Compartment
The characteristics of the Ferranti FL Series Tapchangers are generally similar to those of the mechanisms on the Ferranti DC3 and EFG Series of Tap Changers. The most important feature of the equipment is the single-phase driving motor which is specially designed for the purpose and has certain unique characteristics. The drive is taken through an "Oldham" coupling to a reduction gear unit, the worms and wheels of which are specially designed to give high efficiency and long life. The worm wheel shaft is extended to take the operating handle and the mechanical end stop. The mechanical end stop is of the differential gear type. At the opposite end of the worm shaft wheel is another reduction worm gear unit which operates the timing gear and which also provides the main drive to the selector switch compartments The timing gear camshaft controls the complete sequence of each tap change operation, and all switching operations are linked mechanically to this shaft. The switches are in the form of open contacts under oil. The contacts are spring loaded to the closed position, and they are opened by the operation of cams. The camshaft drives the tap position indicator, and the step switch for the operation of the remote tap position indicator. It also operates the limit switches which are included in the start circuit to prevent the initiation of another tap change in the same direction after the end stop has been reached and also to prevent the overrun of the motor into the mechanical end stop.
The Selector Switches Compartment
The selector switch compartment housing is provided with oil drain valve and filter valve and with connection to the Buckholz protection system of the transformer. An approved form of gas bubble deflector is also included. The barrier panel between the selector switch compartment and the interior of the transformer tank is an epoxy resin casting. The bath tub style barrier board switch base which carries the fixed contacts of the selector switch is a high pressure moulding. On the transformer side of the switch base each contact is tapered to accept a connection which is driven home by means of an "Allen" cap-screw. Reversal of this screw drives the connector off the taper if it should be necessary to remove a tapping lead from a selector switch contact. The proximity of the switch bases to the transformer coils enables extremely short tapping leads to be used, so reducing any risk of failure. High pressure mouldings are also used for the bridge pieces which carry the lead screw bearings and busbar supports and for the lead screw half couplings. The moving contact housing is also a high pressure moulding incorporating the driving nut for the lead screws and the sliding contact for the busbars. The copper contact tips are of the balance pressure line contact self-aligning type. Links connect the sliding contact busbars to the main bus- bars which are carried through an aluminium stress tube to the diverter switch compartment on the top of the porcelain. This stress tube forms the central electrode of the bushing porcelain. As the selector switch contacts do not make or break current, they give trouble-free service for many years. The pocket underneath the selector switch compartment houses a reduction gear unit which is driven from the driving motor compartment, and which is connected through an insulated coupling to drive the selector switch and diverter switch gearing. A Geneva mechanism incorporated in this gearing provides 180° lost motion on reversal. Where three-phase operation is required, the three single- phase units are coupled together by shafts which are driven from the reduction gear unit in the pocket under the selector switch compartment. These shafts can be seen in Figure above. Each reduction gear unit also includes a local tap position indicator which is visible through a window in the pocket. These indicators are intended solely as setting devices to ensure correct contact alignment and correct coupling between the phase units. The tap position indicator in the timing gear box provides the main reference concerning the tapping position.
The Diverter Switch Compartment
The cover of the diverter switch is made of aluminium alloy and it is provided with a special hinge which enables it to be swung away easily by one man. An oil level gauge and expansion chamber fitted with an air release valve is incorporated in the cover and an oil drain valve is provided in the bottom of the diverter switch housing. A special platform is provided to give access to the diverter switch assembly. In the diverter switch assembly all the pins and levers of the toggle and stored energy mechanism of the diverter switches have ample bearing surfaces, and a dashpot is included in the toggle mechanism to absorb any shock from the sudden movement of the system. The diverter switch driving springs are conservatively rated and operate at a very low stress. They are wound in the heat-treated state to minimise any possibility of failure, and, whilst springs are fitted at both ends of the switch, those at one end are sufficient to ensure satisfactory operation. The diverter switch arcing contacts are self-aligning, and both the fixed and moving arcing contacts are surfaced with tungsten copper. Ratchet or servo devices are incorporated to allow all roller contacts to rotate so as to ensure even wear all round the periphery, and contact bounce has been virtually eliminated. The main current carrying diverter switch contacts which close the circuit after a tap change is complete are of the pressure balance line contact type. As these contacts do not make or break current, they operate with minimal throughout the life of the transformer. Bridging resistors of the oil cooled, wire-wound, open type are mounted on accessible copper brackets at the sides of the diverter switches. These resistors are short time rated, but they have a liberal margin to cover the continuous operation which may result from "hunting" when on automatic control.